A Coriolis Vibratory Gyroscope (CVG) comprises a sensitive element suitable for vibrating in at least one primary vibration mode when excited by an excitation device. Such a sensor possesses one or more specific axes, called “sensitive axes,” characterized in that when the inertial sensor is rotated around any instantaneous axis of rotation and when its sensitive element vibrates in a primary vibration mode, then said sensitive element vibrates by the Coriolis effect in a secondary vibration mode with an amplitude proportional to the projection of the instantaneous vector of rotation on said sensitive axis. Said secondary vibration mode is specific to the geometry of the sensitive element, to the primary vibration mode, and to the sensitive axis in question. The vibration amplitude of each secondary mode is measured by a detection device in order to determine the projection of the rotational speed of the vehicle on the sensitive axis which is associated with said secondary vibration mode.
The principle of CVG gyroscopes is a concept particularly suitable for miniaturization and therefore is particularly employed in the production of micro-electro-mechanical system (MEMS) gyroscopes, considering that what is referred to as a micro-electro-mechanical system is a mechanical device comprising one or more movable parts, of which at least one dimension is less than one millimeter and the production of which is carried out by technological methods conventionally used in the field of electronics and microtechnology.
Thus, MEMS-type CVG gyroscopes are particularly well-suited for applications requiring devices of reduced size, such as space applications, the guidance and control of UAVs, etc.
However, a common disadvantage of CVG inertial sensors is the appearance on detection devices of an additional signal, which is representative of the primary vibration only and not of the rotational speed to be measured. This additional signal is detrimental to the proper functioning of the gyroscope, as it degrades the quality of the measurement. It is therefore necessary to implement complex electronic processing for existing CVG gyroscopes in order to eliminate this additional signal and obtain the rotational speed measurement.
The origin of this additional signal can be linked to multiple physical phenomena of different types. One of these phenomena, well known to one skilled in the art, is commonly referred to as “mechanical coupling.” This is an unfortunate coupling between the secondary vibration modes and the primary vibration mode(s) which causes the primary vibration to induce a vibratory motion in the secondary vibration modes and vice versa. The mechanical coupling thus leads to an additional signal appearing in the detection system, even when there is no rotation of the gyroscope.
The origin of the additional signal also can be linked a lack of selectivity in the detection device. Indeed, it is common for the detection devices used to detect secondary vibrations also to be suitable for detecting, even with low sensitivity, the vibration of the primary vibration mode. Thus, an additional signal appears in the detection system in the absence of any rotation of the gyroscope, which requires electronic processing and which degrades the quality of the rotational speed measurements.
The appearance of such an additional signal is a recurring deficiency in the prior art, which the invention aims to overcome.
Most existing Coriolis gyroscopes are single-axis, i.e., they measure the rotation relative to a single sensitive axis. An example of a single-axis CVG gyroscope exploiting the vibrations of a quartz tuning fork planar structure is described in patent application FR 2 789 171. In order to characterize the motion of a vehicle, it is thus necessary to use an assembly of at least three single-axis gyroscopes to determine rotational information in the three degrees of rotational freedom of the vehicle.
One particular family of CVGs groups the monolithic dual-axis CVGs. The use of such an inertial sensor allows obtaining a vehicle's rotational information in two axes, with a single device. However, similarly to single-axis Coriolis gyroscopes, it is necessary to use an assembly of at least one single-axis CVG and a dual-axis CVG to determine rotational information in the three degrees of rotational freedom of the vehicle.
An example of this family of inertial sensors, made of bent sheets, is described in U.S. Pat. No. 6,539,804. In the example cited, the structure of the gyroscope is composed, among other things, of a vibration ring and four non-deformable masses connected to the upper face of the ring by coupling elements. In the operation presented, the secondary vibration mode corresponds to out-of-plane movements of the masses, and the detection system for the secondary modes relies on evaluating the stresses generated in the coupling elements by the out-of-plane movements of the masses using piezoelectric ceramics brought to the surface of said coupling elements. The primary vibration mode is a deformation of the ring, intended to cause the masses to vibrate radially; however, as the masses are located on the upper face of the ring, the radial movement of the masses is necessarily accompanied by an out-of-plane movement similar to the vibration of the secondary modes. This therefore produces a mechanical coupling between the primary vibration mode and the secondary vibration mode that causes an additional signal to appear on the detection device and degrades the rotational speed measurement.
Furthermore, the method in the example cited above is based on die-cutting and bending a steel sheet, which is incompatible with MEMS technologies and therefore not conducive to miniaturization.
Another particular family of CVG gyroscopes includes monolithic tri-axis CVGs. The use of such an inertial sensor allows obtaining a vehicle's rotational information in three axes using a single device, dispensing with the step of assembling single-axis and dual-axis inertial sensors, and thereby gaining further compactness.
Such a tri-axis inertial sensor requires designing a structure excited by one or more primary vibration modes coupled by Coriolis force to at least three different secondary vibration modes. For example, document FR 2 821 422 presents the possibility of creating a planar monolithic structure for a tri-axis gyroscope by monolithically combining two planar sub-elements: one of the sub-elements is a planar structure for a single-axis gyroscope with an out-of-plane sensitive axis (Z axis) and the other sub-element is a planar structure for a dual-axis gyroscope, inspired by document FR 2 741 151, having its two sensitive axes in the plane of the substrate (X and Y axes). The operation of such a gyroscope thus requires the excitation of the primary vibration mode of each of the sub-elements and therefore requires the use of two distinct excitation devices. This leads to electronic excitation systems that can be complex and cumbersome.
In order to reduce the bulkiness, the complexity, and the power consumption of the excitation device of an inertial sensor in the tri-axis CVG family, it is of interest to use only one primary vibration mode, and to couple this primary vibration mode to each of the three secondary vibration modes.
Patent application WO 98 17973 describes a monolithic tri-axis CVG gyroscope, compatible with MEMS technologies and using a single primary vibration mode. However, this gyroscope uses the same system of electrodes to detect the amplitudes of the primary vibration mode as well as two of its secondary vibration modes (corresponding to the X and Y axes). Due to this, by its very design it presents a lack of selectivity of the detection devices, such that the electrical signals representative of the rotation information for the X and Y axes are not only added together, but are also added to an additional electrical signal representative of the vibration mode. It is thus necessary to implement complex electronic processing of the signals issuing from said electrodes in order to eliminate this additional signal issuing from the primary vibration mode and separate out the information representative of the rotational speeds on the two sensitive axes X and Y.
US patent 2010/0236327 A1 describes a monolithic tri-axis gyroscope compatible with MEMS technologies and only using a single primary vibration mode. This gyroscope presents a lack of selectivity of the detection systems. Indeed, in this device, the secondary vibration mode corresponding to the rotation on the X axis corresponds to an out-of-plane movement of the masses 22 and 26. The amplitude of this movement is detected by measuring the variation in the values from the capacitors 82 and 86. However, according to the laws of physics, the values from the capacitors 82 and 86 are also somewhat sensitive to the movements within the plane of the masses 22 and 26 induced by the primary movement. These values from the capacitors 82 and 86 are also slightly sensitive to the rotations of the same masses 22 and 26 during the secondary vibration corresponding to the rotation on the Z axis. As a result, the electrical signal representative of the information relating to rotation on the X axis is added to the additional electrical signals representative of the vibration of the primary mode and of the secondary vibration mode Z. It is then necessary to implement a complex electronic processing of the signals in order to separate the information representative of the rotational speed on the X axis from the additional noise issuing from the primary vibration mode and secondary vibration mode Z.
Moreover, in order to guarantee measurements of rotational speeds with the same accuracy in the three axes, it is important that the gyroscope sensitivities be substantially the same in the three sensitive axes. To this effect, a Coriolis coupling coefficient is defined. It represents the capacity of a sensitive element to vibrate according to a Coriolis force-induced secondary vibration mode when it is excited in a primary vibration mode and is made to rotate about the sensitive axis associated with said secondary vibration mode. The Coriolis coupling coefficient can be quantified by the following relationship:
  K  =                        1        V            ⁢                        ∫                      ∫            ∫                          V            ⁢                                    u            _                    pilote                ⋀                              u            _                    detection                        in which                {right arrow over (u)}pilote is the displacement vector which characterizes the vibration of the primary mode,        {right arrow over (u)}detection is the displacement vector which characterizes the vibration of the secondary mode, and        V is the volume of the structure.        
Obtaining three equivalent sensitivities for the tri-axis gyroscope involves the use of a vibrating structure in which the Coriolis coupling coefficients are substantially identical for the three sensitive axes. A gyroscope having a sensitivity in one axis that is much higher than its sensitivity in another axis cannot truly be used in a satisfactory manner to determine rotational information for its three sensitive axes.